Preloading phase of selenium is because of people with hashimoto thyroiditis who dont respond well to iodine, but if selenium is used for 4-5 weeks than they have no problem, all other folks dont really need to preload- but not many people tested antibodies (for hashimoto), so its safer just to preload.
Selenium improves function of thyroid and fat metabolism, and in combination with iodine improves thyroid function (thus improves metabolism) and also it helps adrenals, both are very good antioxidant- with iodine being probably one of the strongest antioxidants.
L tyrosine can also be very effective and improve energy levels, but it varies from person to person, in some it can cause irritability- in which case is best to drop it... pyridoxal 5’-phosphate seems to improve efficacy and absorbtion.